In closed loop control systems which control the operation of air/fuel forming devices of internal combustion engines such as carburetor and fuel injection systems it is usual to employ a gas sensor to sense a component of the exhaust gases issued from the engine which is indicative of the air/fuel ratio of the combustible mixture being fed therein. In most cases the sensor is an oxygen sensor which uses a solid electrolyte such as zirconium.
Although the above-mentioned zirconium type oxygen sensor (as it will be referred to hereinafter) functions satisfactorily at elevated temperatures, at low temperatures the internal impedance of the zirconium (or equivalent) is so high that the maximum voltage of the output signal therefrom is exceedingly low resulting in the range defined between the minimum and maximum values of the output signal voltage being inadequate to provide accurate control by the closed loop control system. Hence erratic operation of the engine when the closed loop system is supplied with a signal (from the gas sensor) which varies within such a narrow range, is inevitable.